Varnish on Victorian Floor Tiles

We often come across simply stunning Victorian tiled hallways which have been mindlessly daubed with varnish. The kind of varnish that should be used on yachts, wooden floors or even furniture; nothing closely related to clay tiles. But the varnish still finds its way there; the Birmingham area being the worse we have found for this crime.

Not only does the varnish flake, scratch and peel over time, the varnish will often discolour and darken as the ingredients age and break down. On a wooden floor you would sand this off and start on a fresh, but on 100+ year old clay tile floor you cannot do that. So how it is removed?

Maybe we should rephrase the question. How do we remove the varnish from Victorian floor tiles without damaging the tiles or indeed bleaching them to a new colour? The only real answer is with extreme care working in small areas, with pretty horrible chemicals; definitely not for the DIY buff.